V54B-01 16:00h
No Indications from Cr Isotopes for the Late Heavy Bombardment on Earth in Early Archean Metasediments from Isua, Greenland.
The short-lived $^{53}$Mn-$^{53}$Cr (half-life of 3.7 Ma) isotope system has been widely used in cosmochemical studies and excess $^{53}$Cr relative to the terrestrial $^{53}$Cr/$^{52}$Cr ratio has been detected in a variety of ancient solar system objects. Inspired by the highly debatted, apparent indications from W isotopes for a meteoritic component in ~3.8 Ga sediments from Isua ([1]) and the potential of the Mn-Cr extinct isotope system to trace extraterrestrial components in K-T sections ([2]), we have initiated a long-term study of Cr isotope systematics in these sediments, from which Rosing ([3]) reported graphitic microparticles with isotopically light C, indicative of a biogenic origin. We document our ability to reproduce the terrestrial Cr standard to +/- 0.15 \epsilon-units, applying a second order mass bias correction to acccount for residual mass fractionation effects after application of a strict exponential law. While we are able to detect an apparent deficit of $^{53}$Cr in the order of -0.3 \epsilon-units in the K-T boundary clay from Stevns Klint and the carbonaceous chondrite Allende (and thus confirm results by [2]), several pelagic shales (with Cr concentrations from 30-70 ppm) from Isua give statistically indistinguishable values from the terrestrial standard. We are thus not capable of detecting a Cr-component of extraterrestrial origin within the limits of our mass spectrometric precision in the Isua sediments and therefore are unable to confirm the W-isotope evidence for early meteorite bombardment apparently recorded by them. [1] Schoenberg R., Kamber B.S., Collerson K.D. and Moorbath S. (2002), Nature 418, 403-405; [2] Shukolyukov A. and Lugmair G.W. (1998), Science 282, 927-929, [3] Rosing M.T. (1999), Science 283, 674-676.
V54B-02 16:15h
Fractionation Of Silicon Isotopes By Present-Day Demosponges From The Spermonde Shelf, Indonesia.
The silicon and carbon biogeochemical cycles are closely linked since diatoms (phytoplankton with an opal skeleton) transport large quantities of silicon and carbon from the surface- to the deep ocean. The silicon isotopic composition of biogenic opal could provide additional information for climate studies, continental weathering and the secular variations of biogenic silica in seawater. So far, only limited data are available for the fractionation factors between dissolved silica in seawater and biogenic opal. De La Rocha et al. (1997) showed that the fractionation factor between dissolved silica and diatom opal is not species dependent. However, other organisms with opal skeletons, such as sponges and radiolarians, have not been investigated in detail so far. We selected three demosponge species from a tropical region (Spermonde Shelf, Sulawesi, East Indonesia), where temperature differences are minimal. Therefore, silicon isotope fractionations induced by temperature, or differences in the dissolved silica isotopic composition are unlikely. The demosponges of this study display a range of $\delta$$^{30}$Si between -2.0 and +0.5$\permil$. This range largely overlaps with previous published data, but extends the range to heavier values. The three species show different $\delta$$^{30}$Si averages: {\it Petrosia hoeksemai}: -0.04 $\pm$ 0.14$\permil$; {\it Stylissa carteri}: 0.20 $\pm$ 0.27$\permil$ and {\it Clathria thalysias basilana}: -1.28 $\pm$ 0.26$\permil$. The results from the studied demosponges suggest that fractionation factors are species dependent, since the sponges display systematic differences at a single locality. We did not observe correlations between $\delta$$^{30}$Si, opal content and water depth. Sponges are probably dissolved silica limited and not food limited in their growth. Consequently, different biological mechanisms for the uptake of dissolved silica and the conversion to sponge opal could be the reason for the different fractionation factors for each species. References De La Rocha et al., (1997) Geochim. Cosmochim Acta 61, 5051-5056.
V54B-03 16:30h
Silicon Isotopic Composition of Isua BIF and Other Early Archean Supracrustal Rocks: a Tracer for Early Life?
High temperature vapor condensations and stardust forming stellar outflows strongly differentiate Si-isotopes (-650\permil$<$\delta$^{29}$Si$<$+200\permil). In contrast, on Earth, the major cause of slight Si isotope fractionation (-1.8\permil $<$\delta$^{29}$Si $<$+1.5\permil) is related to preferential biological uptake of $^{28}$Si by diatoms, radiolarian, sponges and plants in building their opaline frustules, spicules and phytoliths. As a consequence, modern waters, clays, soils and sediments are also fractionated: -0.9\permil$<$\delta$^{29}$Si$<$+1.7\permil, while Phanerozoic magmatic and metamorphic processes leave terrestrial crystalline rocks almost unfractionated: -0.4\permil $<$\delta$^{29}$Si $<$+0.2\permil. Spectacular claims for discovery of oldest evidence for terrestrial life in early Archean (ca 3.7-3.8 Ga) metamorphosed rocks from southern West Greenland have been challenged in a series of studies, and there is need for some independent tracer to gain new insights into the quest for primitive life. In order to unravel potentialities of silicon isotopes, we determined the Si-isotope composition of four groups of rocks from the Isua Greenstone Belt: magnetite-quartz "Banded Iron Formations" (BIF); mica-feldspar-quartz-(garnet) schists of potential pelitic origin; a series of tonalitic gneisses, metabasalt pillows, volcanogenic sediments and hydrothermal quartz veins and vesicles. Specimens were powdered using diamond-coated microdrills. Si was purified by TEA molybdate co-precipitation and measured on a Nu Plasma MC-ICP-MS using Mg external doping in dry plasma mode following Cardinal et al's (2003) methodology. Measured $^{29}$Si/$^{28}$Si are expressed with the \delta$^{29}$Si notation relative to the NBS28 quartz standard. Overall reproducibility assessed on 5 BIF duplicates is better than 0.08\permil. Merck Quartz aliquots processed in the same way as the Isua specimens are unfractionated, showing that our chemical procedure does not produce any isotopic artefacts. Metasediments yield an undifferentiated signature: -0.15\permil $<$\delta$^{29}$Si $<$+0.00\permil. Since they may reflect the average composition of pre-3.8Ga Archean crust, this points to a lack of significant "stellar dust" or "impact-related thermal volatilisation" imprints from pre-3.8Ga meteoritic infall on detrital Si-isotope compositions. The undifferentiated signature (-0.06\permil$<$\delta$^{29}$Si $<$+0.21\permil) of magmatic and hydrothermal derived rocks confirms that Archean crystalline rock-forming processes compare with their Phanerozoic counterparts in terms of their inefficacy to fractionate silicon isotopes. BIF's differ by their significant relative $^{29}$Si depletion, which is stronger for the well-preserved, finely-banded BIF (-1.28\permil$<$\delta$^{29}$Si$<$-1.11\permil) than for those with irregular or folded banding (-0.71\permil$<$\delta$^{29}$Si$<$-0.28\permil). Moreover, the finely banded BIF presents very homogeneous interlayer \delta$^{29}$Si. Considering the seawater-derived origin of Isua BIF (e.g.Bolhar et al, 2004), the $^{29}$Si depletion might either directly reflect the ambient early Archean ocean or seawater-derived hydrothermal fluid, or indicate that BIF precipitation was associated with some unknown organic or inorganic fractionation process. Because the observed isotopic differentiation fits in the range of modern biogically-mediated Si-isotope fractionations, the hypothesis of some potential links between BIF precipitation and early biological activity cannot be overlooked.
V54B-04 16:45h
Coupled Fe and S Isotope Evidence for Archaean Microbial Fe(III) and Sulphate Reduction
The development of transition metal stable isotope geochemistry over recent years, fuelled by advances in analytical technology, has permitted new approaches to established geochemical problems. One such application is the understanding and tracing of early Earth biogeochemistry, with a particular emphasis on the activities of primitive microbial life, where evidence from traditional isotope systems has so far been equivocal. The earliest forms of microbial respiration are thought to involve sulphate or Fe (III) reduction, $^{e.g. 1,2}$ or perhaps both, and as such combined Fe-S isotope systems provide a potentially powerful tool to trace these processes. Here we report a Zn-Fe-S data set from sulphide and pyrite grains from the 2.7 Ga Belingwe Belt, associated with sulphate reducing microbial communities. Analyses of individual mm size sulphide and pyrite grains show large depletions of light Fe isotopes, as well as a large variation in Fe isotope composition with a range of -0.7 to -2.7 \permil. Furthermore these variations are correlated with depletions in light S isotopes measured from the same samples, which range in isotope composition from -3 to -18 \permil $^{3}$. Zn isotopes also show significant positive fractionations, up to 0.8\permil, particularly in organic rich black shales. The most striking feature of our dataset is a tight correlation between sulphur and iron isotopes. This relationship is most readily explained in terms of a reducing sedimentary environment. In modern anoxic sediments dissimilatory bacterial Fe (III) reduction, and further down the sediment column, sulphate reduction produce solubilised light Fe$^{2+}$ and S$^{2-}$, which in solution together immediately react to form isotopically depleted sulphides and ultimately pyrite. Experimental constraints$^{4}$ and measurements of natural pyrite from modern sedimentary settings (S. Severmann, pers comm) demonstrate that the solid sulphide produced is light in Fe. We will present a quantitative diagenetic model which demonstrates that, in a closed system coupled Rayleigh depletion in the light isotopes of both Fe and S by these processes down the sedimentary column could produce a correlated Fe and S dataset. Quantitatively, this process requires the sulphate in the pore water to become significantly depleted, a process that generally does not occur today because of high pore-water sulphate levels$^{5}$ but is much more likely in the Archaean where seawater sulphate levels were probably two orders of magnitude lower. We note that both Fe and sulphate reduction as late as 2.7 Ga is indicated by SSU rRNA phylogenetic models, but our approach has potential applications further back in time where such constraints are lacking. $^{1}$ Y. Shen and R. Buick, Earth Sci. Rev., 2004, {\bf64}, 243-272 $^{2}$ M. Vargas et. al., Nature, 1998, {\bf395}, 65-67 $^{3}$ N.V. Grassineau et al., 2001, Proc. Roy. Soc. London B, {\bf268}, 113-119 $^{4}$ I. Butler et al., 2003, Geochim. Cosmochim. Acta, {\bf67}, A51 $^{5}$ S. Severmann et al, 2004, this volume
V54B-05 17:00h
Behavior of Boron Isotopes During Chemical Weathering: a Global Approach
Boron has two isotopes ($^{10}$B and $^{11}$B) that fractionate largely during Earth surface processes. The major fractionating step takes place during low temperature water-rock interactions. Up to 20-30 \permil difference in $\delta$$^{11}$B units are shown to occur during the adsorption of boron onto surfaces or its precipitation into solids. Light boron has a much greater affinity for neoformed solids while the residual solution is enriched in heavy boron. For example, seawater has a boron isotopic composition of 40 \permil, mainly due to the preferential removal of $^{10}$B during oceanic seafloor weathering, adsorption onto fluvial sediments and chemical weathering reactions occurring in soils. The high sensitivity of boron isotopes fractionation to water rock interactions make it a valuable tool to constrain chemical weathering processes. To have a global picture of the behavior of boron isotopes during chemical weathering of rocks at the surface of the continents, we analyzed the largest rivers for both the dissolved and suspended load. Dissolved boron isotopic compositions were published earlier ([Lemarchand et al., 2000]) and we here focus on the results for the suspended load and for the bottom sands. Boron clearly appears as a mobile element when its abundance in suspended sediments is normalized to upper continental material. Boron depletion in suspended solids is climate dependent, with higher depletion is warm climates. On average, more than 50 % of boron is transported to the ocean in a solid form. While the dissolved load of boron is clearly enriched in $^{11}$B (0 \permil) with respect to the mean upper continental crust (-10 \permil), isotopic composition of the suspended load of major rivers does not differ significantly from that of the continental crust. This result indicates that suspended material is not significantly fractionated with respect to the continental crust by chemical weathering processes or that the mass budget of boron partitioning between solids and water does not allow the residual solids to be significantly different from bedrock. This is supported by a Rayleigh distillation model. Our boron data, both in concentration and isotopic composition, give strong support to the idea that shale erosion is a major source of suspended sediments in large river system, making thus boron isotopes a good tracer of intra-continental recycling. Lemarchand et al., 2000, Nature, vol 408, pp 951-954.
V54B-06 17:15h
Iron Isotope Constraints on the Archean and Paleoproterozoic Ocean Redox State
The rise of atmospheric oxygen level, which started by 2.3 Ga, was one of the most dramatic environmental changes in the Earth's history. Since Fe, along with C and S, are coupled with and maintain the redox state of the surface environment, Fe seawater concentration and isotopic composition were likely affected by the change in the redox state of the atmosphere. Study of S isotope composition of sedimentary sulfides over geological time has placed important constraints on the S cycle and the evolution of ocean chemistry and here we apply a similar time-record approach to explore potential changes of Precambrian Fe oceanic cycle. We analyzed Fe isotope compositions of ~ 150 pyrites from 20 black shale units, specifically focusing on Late Archean to Paleoproterozoic time. $\delta$$^{56}$Fe values of handpicked sulfides were obtained using a Neptune MC-ICPMS at WHOI and are reported relative to IRMM-14 with an external precision of 0.1$\permil$ at 2$\sigma$ level. The emerged general pattern of Fe isotope record allows dividing the Earth's history into four stages which are strikingly similar to the stages defined by the $\delta$$^{34}$S, $\Delta$$^{33}$S as well as other indicators of the redox state of the atmosphere and ocean (1) Stage 1 from before 2.8 to 2.45 Ga is characterized by highly variable and negative $\delta$$^{56}$Fe values of pyrite. The entire range between 0.22 to -3.5$\permil$ is often observed within single section of black shales but individual pyrite nodules from the same stratigraphic level have similar $\delta$$^{56}$Fe values. These extremely variable and low $\delta$$^{56}$Fe values are interpreted to reflect the reservoir effect during partial oxidation of hydrothermally-derived Fe(II). Because the Fe oxide sink associated with BIFs preferentially remove Fe heavy isotope, an enrichment in Fe light isotope for the residual oceanic Fe(II) pool is expected and is recorded during pyrite formation and burial in Black Shales. (2) Stage 2, representing a transition period, covers the time interval from 2.45 to 2.2 Ga and is characterized by a smaller range of negative $\delta$$^{56}$Fe values (above -1.8$\permil$) and by positive $\delta$$^{56}$Fe values up to 1.0$\permil$. Surprisingly, these isotopic signatures reflect changes in the oceanic Fe cycle on the same stratigraphic level where the most sensitive indicators for the rise of atmospheric oxygen appear. (3) Stage 3 from 2.2 to ~1.6 Ga ago, is characterized by $\delta$$^{56}$Fe values ranging from -0.3 to 1.2$\permil$ . The ubiquitous positive $\delta$$^{56}$Fe values in 2.4 to 1.8 Ga black shales are striking and might be related to the increased effect of sulfide precipitation in a redox-stratified ocean. (4) Stage 4 from 1.6 Ga through most of the phanerozoic is characterized by sedimentary pyrite having a limited range of $\delta$$^{56}$Fe variations (less than 0.5$\permil$ around igneous value at ~0$\permil$). Because pyrite formation in modern organic-rich marine sediments is mediated by sulfate-reducing bacteria and proceed through the reduction of Fe-oxides to Fe(II), this rather limited variability of $\delta$$^{56}$Fe values suggests that bacterial reduction of Fe-oxides is unlikely to explain the extreme light $\delta$$^{56}$Fe values found during the Archean.
V54B-07 17:30h
Deep Marine Sediment Diagenesis of Germanium, Silica, Lithium and Lithium Isotopes in ODP-177: The "Missing Oceanic Ge Sink"
Understanding sediment diagenetic alterations of the crustal Ge/Si ratio is important as a clue to identifying the phases and processes responsible for burying the "missing" portion of the input Ge-flux (rivers and MOR vents) that can not be ascribed to diatom Ge/Si burial. In the ocean, in diatoms, and in most low-temperature biogeochemical processes, Ge behaves much like a trace cogenor of Si, substituting for Si and displaying Ge/Si ratios that don't deviate much from crustal (10-6 mol/mol). Significant fractionation of the ratio is now recognized for continental weathering processes and hydrothermal basalt reactions. During early diagenesis in the shallow suboxic zone of marine sediments, where non-silicate phases may dominate Ge-cycling (e.g., FeOOH), pore water [Ge] also reflects uptake into authigenic phases but seldom exceeds several hundred picomolar (10-12 Molar). In this paper we present the first deep pore water Ge data from Sub-Antarctic South Atlantic Sites drilled during ODP Leg 177 across the Southern Ocean biosiliceous ooze belt. In all sites (ODP 1088-1094), pore water Ge displays steep gradients and maxima and minima that are unrelated to opal diagenesis. Deep maxima Ge-concentrations approach 120 nM, 1000-fold higher than seawater (0.1 nM). The shapes of these profiles require local diagenetic sources and sinks for Ge that are very large, and both upward and downward fluxes that far exceed those estimated for local Ge-burial in diatomaceous opal. Thus there must be other non-biosiliceous phases that carry Ge to the seafloor that are activated during deep burial diagenesis. Temperature does not seem to be a factor: deep borehole temperatures do not exceed 46-deg C. So far we have been unable to identify these reactive phases, although we suspect some combination of chert formation and clay alteration as likely culprits. In contrast, Si profiles are featureless, exhibiting near-zero Si gradients over most intervals, with Si typical of opal saturation (1-2 mM) diluted by opal/detrital sediment ratio. Pore water lithium [Li] and del-7Li display both uptake and release profiles related to aluminosilicate clay formation and destruction. Del-7Li varies from +5 to minus 25 per mil (ref seawater), while [Li] varies from sligtly below seawater values (25 uM) to about 250 uM. The downcore data are consistent with release of Li from clay minerals carrying light (7Li depleted) cations. These Li and Li-isotope data are internally consistent, but do not shed any light on the diagenetic processes affecting deep Ge diagenesis.
V54B-08 17:45h
New technical developments in silicon isotope measurements by MC-ICP-MS
The fractionation of silicon (Si) isotopes during biological uptake and opal biomineralization allows silicic acid utilization in aquatic environments to be quantified from the geological record. The use of this direct nutrient proxy has been hampered by analytical difficulties in the measurement of Si isotopes in biogenic silica. The recent development of high-resolution multi-collector ICP-MS and novel sample introduction techniques now allow for the rapid and precise measurement of Si isotope abundances from very small amounts of biogenic opal. However, interferences and matrix effects can make Si isotopic measurements by MC-ICP-MS difficult, and considerable care must be taken to overcome these technical difficulties. Refined techniques also allow for the measurement of small samples of silica and silicate minerals to be analysed in a routine fashion, enabling the study of Si isotope fractionation between different mineral phases in terrestrial and extra-terrestrial materials. Using the Nu1700 high resolution MC-ICP-MS allows for the resolution of all significant interferences during measurements, and matrix effects have been studied for a range of sample introduction systems and chemical preparation methods allowing for different running conditions. We will review current techniques and analytical capabilities for the measurement of Si isotope ratios using MC-ICP-MS and show how these can be applied to a range of silicate mineral phases. Results will focus on the Si isotope composition of biogenic opal from cleaned diatom frustules and also on water samples. The developed techniques allow for high precision measurements to be obtained, which require re-examination of the stability of monomeric silicic acid solutions and ways of storing standard solutions needed for intra- and inter-laboratory calibrations of silicon isotopic analyses.